1. Field of the Invention
The present invention relates to a cleaning method and a polishing apparatus employing such cleaning method, and more particularly to a cleaning method suitable for cleaning substrates that require a high degree of cleanliness, such as semiconductor wafers, glass substrates, or liquid crystal displays, and to a polishing apparatus employing such cleaning method.
2. Description of the Related Art
As semiconductor devices have become more highly integrated in recently years, circuit interconnections on semiconductor substrates become finer and the distances between those circuit interconnections have become smaller. One of the processes available for forming such circuit interconnections is photolithography. In the case where circuit interconnections are formed by the photolithography or the like, it requires that surfaces on which patterns images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor substrates flat for photolithography. One customary way of flattening the substrates of the semiconductor substrates is to polish them with a polishing apparatus. As shown in FIG. 8, a conventional polishing apparatus 76 comprises a turntable 72 having a polishing cloth 70 thereon, and a top ring 74 for holding a semiconductor substrate W and pressing the semiconductor substrate W against the turntable 72. In the polishing apparatus, a chemical mechanical polishing (CMP) of the substrate is performed by a combination of chemical polishing with an abrasive liquid and mechanical polishing with abrasive particles contained in the abrasive liquid. An abrasive liquid supply nozzle 78 is provided above the turntable 72 to supply the abrasive liquid Q to the polishing 70. Further, a dressing device 80 is provided to regenerate, i.e., dress, the polishing cloth 70.
FIG. 9 shows a CMP unit which is constructed as an integral unit having the polishing apparatus 76 shown in FIG. 8 and various devices associated with the polishing apparatus 76. The CMP unit has a substantially rectangular shape in plan, and the polishing apparatus 76 is disposed at one side of the CMP unit, and load and unload units 84a, 84b for placing wafer cassettes which accommodate semiconductor substrates to be polished are disposed at the other side of the CMP unit. Transfer robots 86a, 86b are movably provided between the polishing apparatus 76 and the load and unload units 84a, 84b so that the transfer robots 86a, 86b are movable along a transfer line C. Reversing devices 88a, 88b for reversing a semiconductor substrate are disposed at one side of the transfer line C, and cleaning apparatuses 90a, 90b, 90c for cleaning the semiconductor substrate are disposed at the other side of the transfer line C. A pusher 10 is disposed adjacent to the turntable 72 to transfer the semiconductor substrate between the top ring 74 and the pusher 10 by vertical movement thereof.
In the polishing apparatus 76 having the above structure, the semiconductor substrate W is held by the lower surface of the top ring 74 and pressed against the polishing cloth 70 on the turntable 72. The abrasive liquid Q is supplied from the abrasive liquid supply nozzle 78 onto the polishing cloth 70 and retained on the polishing cloth 70. During operation, the top ring 74 exerts a certain pressure on the turntable 72, and the surface of the semiconductor substrate held against the polishing cloth 70 is therefore polished in the presence of the abrasive liquid Q between the surface of the semiconductor substrate W and the polishing cloth 70 by a combination of chemical polishing and mechanical polishing while the top ring and the turntable are rotated. The abrasive liquid Q contains various abrasive particles, and the pH of the abrasive liquid Q is adjusted in accordance with the kind of semiconductor substrates to be polished.
As described above, as semiconductor devices have become more highly integrated, circuit interconnections on semiconductor substrates become finer and the distances between those circuit interconnections have become smaller. Therefore, in the above polishing process, if a particle greater than the distance between interconnections adheres to a semiconductor substrate and thus such particle remains on the product, i.e., semiconductor device, then the particle will short-circuit interconnections on the semiconductor device. Therefore, any undesirable particles on the semiconductor substrate have to be sufficiently smaller than the distance between interconnections on the semiconductor substrate. Such a problem and a requirement hold true for the processing of other substrates, including a glass substrate to be used as a mask, a liquid crystal panel, and so on.
In the above-mentioned CMP process, the semiconductor substrate which has been polished is transferred to the cleaning apparatuses 90a, 90b, and 90c. In the cleaning apparatuses 90a, 90b and 90c, for example, a scrubbing cleaning process in which a cleaning member, such as a brush or a sponge, is used to scrub a surface of the semiconductor substrate while supplying a cleaning liquid, such as pure water, and a spinning dry process subsequent to the scrubbing cleaning process are preformed, and the abrasive particles or the ground-off particles attached to the semiconductor substrate during the polishing process are removed from the semiconductor substrate.
When pure water (deionized water) is supplied to the semiconductor substrate which has been polished, the pH of the abrasive liquid remaining on the semiconductor substrate changes greatly. Therefore, in some cases, abrasive particles which have been dispersed in the abrasive liquid having an original pH are aggregated together, and adhere to the surface of the semiconductor substrate. For example, in slurry of colloidal silica which is generally used for polishing a SiO2 layer, silica particles which are abrasive particles are stable in alkali solution having a pH of about 10, and form secondary particles having a diameter of about 0.2, μm due to aggregation of primary silica particles. If this slurry is rapidly diluted with pure water to lower the pH of the slurry to 7 or 8, then the electric potential on the surfaces of silica particles is rapidly changed by so-called pH shock, and the silica particles become unstable to thus aggregate the secondary particles to form larger aggregates. In this specification, the pH shock is defined as a rapid change of a pH. This holds true for the dressing process of the polishing cloth 70. To be more specific, when pure water as a dressing liquid is supplied onto the polishing cloth 70 holding the abrasive liquid Q thereon, the pH of the abrasive liquid is rapidly lowered to cause abrasive particles to aggregate. These aggregates remain on the polishing cloth 70 and cause the semiconductor substrate to form scratches in the polishing process.